This invention relates to ignition systems, and in particular to an ignition system which employs two energy sources, the first to create a spark and the second to sustain an arc.
The goal of any ignition system is to ignite an air/fuel mixture such that a self-sufficient combustion process is initiated after the arcing has stopped. Air/fuel mixtures close to stoichiometric require very little ignition energy to generate a self-sustaining flame kernel. However, generating a self-sustaining flame kernel becomes more and more difficult as the air/fuel ratio deviates further and further from stoichiometric or as the air/fuel mixture becomes diluted with exhaust gas recirculation.
Both Coil Ignition (CI) and Capacitive Discharge Ignition (CDI) systems use one energy storage device to create the spark and to sustain the arc. Problem arise when most or all of the stored energy is consumed to create the spark and no energy is left to sustain an arc. This occurs at certain engine speeds and load ranges. Further problems with CI systems are that they store their energy in a transformer making it an inefficient transformer, and they try to transfer all of their stored energy through this inefficient transformer. The main advantage of Capacitive Discharge Ignition (CDI) is the quick rise time of the very high voltage which immediately breaks down the spark gap, preventing the voltage from slowly dissipating in the circuit. This provides the ability to fire fouled plugs or larger gaps.
Breakdown Ignition (BDI) systems are identical to CDI systems, but include a capacitor in parallel with the spark gap. This capacitor stores energy that is being expended on creating the spark. This stored energy is quickly dissipated upon spark creation in the form of high current arc. There are several problems with this configuration, however. First, the presence of the capacitor increases the rise time of the very high voltage spike, which can cause misfires. Second, the capacitor deprives the spark creation process of energy. To insure that this does not cause misfires, more energy must be stored in the primary capacitor. Efficiency suffers from attempting to force all stored energy in the primary through an inefficient transformer and from having one capacitor charge another capacitor. Finally, the energy requirements for igniting a lean mixture are inversely proportional to the storage characteristics of the capacitor in the secondary. This is because more energy is required to ignite a lean mixture at low pressure while the voltage required to create a spark is lower at low pressures. Since energy can be expressed as 1/2CV.sup.2, it can be seen that less energy is stored for a lower breakdown voltage.
Supplementary Secondary Energy (SSE) ignition systems have one energy source for the spark and another for the arc in an effort to lengthen arc duration. These systems are basically CDI systems with additional stored energy in the secondary which is discharged upon spark creation. Existing SSE systems are inefficient because the secondary energy discharges through the secondary winding of the transformer, thereby charging the primary capacitor. Examples of such systems are disclosed in U.S. Pat. Nos. 4,136,301 to Shimojo et al. and 4,301,782 to Wainwright. In the '782 patent, an attempt at isolating the discharge path is disclosed, but the method involves placing an inductor in the discharge path. Including an inductor or a resistor (as in U.S. Pat. Nos. 4,345,575 to Jorgenson and 4,269,161 to Simmons) decreases the peak current which dims the arc intensity.
One object of this invention is to improve the ignition process. In particular, one object of this invention is to maximize efficiency by separating the ignition process into two phenomena, the spark and the arc. Another object of this invention is to achieve maximum power transfer of ignition energy from the spark source to the spark gap by better matching the impedance of the spark plug to the impedance of the spark source.